1. Field of the Invention:
This invention relates to a fuse device for cutting off an overcurrent and, more particularly, to a fuse device, in which a fuse element is accommodated together with an arc-extinguishing agent in a casing.
2. Description of the Prior Art:
In a fuse device, in which a fuse element consisting of a fusible metallic material is accommodated in a casing, the temperature inside the casing is locally elevated, although momentarily, to several thousand degrees by an arc current that is caused when the fuse element is broken apart. In this case, it is liable that the casing is destroyed explosively due to an increase in the inner gas pressure, and such an explosion will cause a fire.
In order to prevent the explosion of the casing at the time of the breaking of the fuse element as noted above, it is tried to seal silica gel particles as arc-extinguishing agent in the casing in case of a largesize large-capacity fuse device with a rated current of several hundred amperes used for a power source apparatus of electric cars. The silica gel particles accommodated as the arc-extinguishing agent together with the fuse element in the casing, is partly fused by the heat generated at the time of the breaking of the fuse element, thus absorbing the arc heat.
However, the prior art arc-extinguishing agent noted above is in the form of particles, so that is leaves continuous interstices among the particles. Therefore, when the fuse element is fused by an overcurrent, its fused portion will not be dispersed into intimate contact with individual particles but remain in inter-particle interstices. In such a case, the arc-extinguishing agent consisting of the silica gel particles is not be contacted by the fused portion of the fuse element over a sufficiently broad area so that it is not fused sufficiently. That is, it can not absorb the arc heat effectively. In addition, the fused portion of the fuse element that is not dispersed sufficiently but is collected in the inter-particle interstices, is liable to form a current short-circuit path by being fusedly attached to particles in the inter-particle interstices. In this case, the function of the fuse device is lost.
In a further aspect, in a small size fuse device with a rated current of several amperes or below, the fuse element consists of a small diameter filament which can be readily broken apart by a physical force. Therefore, in the manufacture of the fuse device the fuse element is liable to be given damage by the silica gel particles that are accommodated as the arc-extinguishing agent in the casing.